The invention concerns a detector device, in particular an anti-theft device.
The device of the invention comprises a sensing means connected to the input of an electronic circuit for triggering a signalling means, and is characterized in that the electronic circuit comprises a continuous differential amplifier whose two inputs (-) and (+) are connected to the terminals of the sensing means, said input terminals also being connected by way of at least two switching means to voltage sources such that, by closing one of the switching means, the polarity of the input terminal in question is inverted, to disable the detection device, and that, by closing the other switching means, with the same sign there is an increase in the polarity of the input terminal in question, for triggering the signalling means.
In accordance with another feature of the invention, each input terminal is connected by way of at least one switching means to the polarity having the same sign of a voltage source, such that closing one of the switching means connected to one of the inputs causes disabling of the device, and that the other switching means connected to the other input causes triggering of the signalling means.
In accordance with another feature of the invention, the switching means for disabling the device comprises, in its connection to the corresponding input of the amplifier, a resistance-capacitance circuit with a substantial time constant, for ensuring slow charging of the capacitance, to cause closure of the switching means to be maintained for a given period.
In accordance with another characteristic of the invention, the capacitance of the disabling circuit, charging of which occurs with a high time constant, is arranged in a discharge circuit comprising a diode and having a longer discharge time constant than the charging time constant, to ensure time delay of the disabling action.